The present invention relates generally to optical communications, and more particularly, to sleep control for energy efficiency in Ethernet passive optical networks.
Owing to the near approach of optical fibers to the users and the passive nature of the remote node, passive optical networks (PONs) consume the smallest energy per transmission bit among various access technologies including WiMAX, FTTN, and point to point optical access networks. Nevertheless, it is still desirable to further reduce energy consumption of PONs since every single watt saving will end up with overall terawatt and even larger power saving as PON is deployed worldwide. Reducing energy consumption of PONs becomes even more imperative for 10G EPON system which provides ten times of the data rate of 1G EPON system owing to the fact that large data rate provisioning requires high power consumption of both the optical and electrical components of the device.
Since traffic of optical network units (ONUs) is rather bursty and changes dynamically, putting ONUs into sleep mode when the ONU does not have downstream or upstream traffic can save a significant amount of ONU energy. Ideally, it is desirable that an ONU stay in sleep mode with low power consumption when the ONU does not have traffic, and switch back into the active mode when traffic of an ONU arrives. However, the broadcast nature of the EPON downstream transmission disallows an ONU entering into the sleep mode. As shown in FIG. 1, the downstream data traffic of EPON ONUs is TDM multiplexed onto a single wavelength, and broadcasted to all ONUs. An ONU has to receive and check all downstream packets, and then decide whether the packets are destined to itself. Since all incoming downstream packets have to be checked, an ONU needs to stay awake all the time to avoid missing its downstream traffic.
A number of schemes have been proposed to address the downstream challenge so as to reduce the energy consumption of ONUs. These proposed energy saving schemes can be divided into two major classes. The first class tries to design proper MAC control scheme to covey downstream queue status to ONUs, while the second class focuses on investigating energy-efficient traffic scheduling schemes. Examples of schemes of the first class are the two-way or three-way handshake processes performed between optical line terminal OLT and ONUs. Typically, an OLT sends a control message notifying an ONU that its downstream queue is empty; the ONU optionally enters into the sleep mode and then sends a sleep acknowledgement or negative acknowledgement message back to OLT. While an OLT is aware of the sleep status of ONUs, it can buffer the downstream arrival traffic until the sleeping ONU wakes up.
However, to implement the handshake process, EPON MACprotocol, i.e., multipoint control protocol (MPCP) defined in IEEE 802.3ah or IEEE 802.3av, needs to be extended to introduce new MPCP protocol data units (PDUs). In addition, the negotiation process takes at least several round trip times, which implies that an ONU has to wait for several round trip times before entering into the sleep status after it infers that its downstream queue is empty. This may significantly impair the energy saving efficiency.
Energy saving schemes of the second class tackle the downstream challenge by designing suitable downstream bandwidth allocation schemes. Formerly, it was proposed by others to implement fixed bandwidth allocation (FBA) in the downstream when the network is lightly loaded. By using FBA, the time slots allocated to each ONU in each cycle are fixed and known to the ONU. Thus, ONUs can go to sleep during the time slots allocated to other ONUs. However, since traffic of an ONU dynamically changes from cycle to cycle, FBA may result in bandwidth under- or over-allocation, and consequently degrade services of ONUs in some degree. Another prior work proposed to schedule the downstream traffic and the upstream traffic simultaneously. An ONU stays in awake status during the upstream time slots allocated to it and switches into sleep status in other time slots. Since the downstream traffic of an ONU is sent over the time slots that its upstream traffic is sent, the ONU stays in awake status during that time period and will not miss its downstream packets. This scheme works well when traffic in the upstream and downstream are symmetric. Yet, it may cause inefficient bandwidth utilization when the downstream traffic outweighs upstream traffic.
Accordingly, there is a need for sleep control for energy efficiency in Ethernet passive optical networks.